Merge Facebook pages

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Hi, I've an older FB page that hasn't been used for a while and a FB page for a new (and similar service) that I'm offering. I'd like to merge the two pages so to carry over some of the followers/content ready for the 'relaunch'. I've followed the guide on FB but I get the following message:

These Pages can't be merged because they're part of different Business Manager accounts.

Does anybody have any idea as to what I need to change to make the merge happen?

Thanks
Mike